There are vast reserves of heavy oil contained in subterranean formations whose natural characteristics such as high viscosity, low API gravity and high molecular weight result in the absence of primary production due to lack of natural reservoir energy. In addition, these reservoirs have relatively low permeability although they may have high porosity.
Attempts have been made to employ known oil displacement procedures for tertiary recovery of the oil by fluid injection through the formation between injection and production wells. These include waterflooding, miscible flooding, chemical or micellar flooding, polymer flooding, thermal recovery by ht fluid injection, thermal recovery by in-situ combustion, solvent flooding, etc., which are well known to those skilled in this art for improving the mobility of the oil, thereby enhancing the recovery of heavy oil.
Among these recovery techniques, the miscible flooding process using carbon dioxide has often been used for enhanced oil recovery. The CO.sub.2 miscible process is often applicable to high gravity crude oils, e.g., 25 degree API, and involves high operating pressures (starting at about 1500 psi and upward). These particular conditions under which the process is practiced produce true miscibility of oil or hydrocarbon with CO.sub.2 thus forming a single phase.
At present, there is increasing interest in the application of carbon dioxide gas for reducing the viscosity of heavy oils thus causing a decrease in the effect of viscous forces associated with the in-situ recovery of heavy oils.
One of the most widely used supplemental recovery techniques is waterflooding which involves the injection of water into an oil-containing formation. As the water moves through the formation, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered. It has also been proposed to add surfactants to the injected water to lower the oil-water interfacial tension and/or alter the wettability characteristics of the formation rock to enhance recovery of the oil. Various surfactant waterflooding techniques are disclosed in U.S. Pat. No. 3,469,630 to Hurd et al and U.S. Pat. No. 3,977,470 to Chang.
Another waterflood technique is taught in U.S. Pat. No. 3,757,861 to Routson which discloses the introduction into the formation of an aqueous solution of peroxide, typically hydrogen peroxide, and thereafter or simultaneously introducing an aqueous solution of alkali metal hydroxides or carbonates or ammonium hydroxide.
U.S. Pat. No. 3,532,165 discloses a method of generating in-situ CO.sub.2 in an oil-bearing formation by injecting therein aqueous solutions of hot sodium carbonate or bicarbonate and sodium sulfite or bisulfite. Thereafter, the formation is subjected to a fluid drive such as water-steam and oil is recovered from the formation.
In this process, a slug of a hot aqueous solution containing sodium hydroxide and sodium bicarbonate is injected into a silica-containing formation which reacts with the silica to form metasilicate, carbon dioxide and thermal energy to decrease the viscosity of the in place oil and reduce the residual oil saturation. Thereafter, the formation is subjected to a fluid drive such as waterflooding to effectively recover the oil reduced in viscosity from the formation.